Forbidden Photo-Ionization and Electron Spin Polarization

A new mechanism involving normally forbidden processes, namely spin-dependent electric dipole and spin-dependent magnetic quadrupole transitions, is applied to the photo-ionization of atoms with a single $\mathrm{ns}$ outer electron. It is shown that, if the incident light is circularly polarized, the spin of the electrons ejected by these forbidden processes gets polarized along the direction of the light propagation as the axis of spin quantization. The cross section of photo-ionization by these transitions is shown to be at its maximum when the propagation vectors of the ejected electrons are parallel to the quantization axis, and is computed for the case of H, Li, and Na. This is in contrast to the spin-independent multipole transitions, where the cross section is zero for electrons ejected in that direction. The photo-ejected electrons are shown to be 100% spin-polarized when they propagate along the quantization axis. However, the spin polarization dropsrapidly when the electron propataion vector deviates from that axis. This is illustrated by calculations on H, Li, and Na.